Wireless communication systems may be multiple-access systems capable of supporting multiple users by sharing the available system resources. Examples of such multiple-access systems include Code Division Multiple Access (CDMA) systems, Time Division Multiple Access (TDMA) systems, Frequency Division Multiple Access (FDMA) systems, and Orthogonal FDMA (OFDMA) systems. In these wireless communication systems, there are any number of base stations that can support communication for any number of user equipments (UEs). Each UE may communicate with one or more base stations via transmissions on the downlink and uplink. The downlink (DL) refers to the communication link from the base stations to the UEs, and the uplink (UL) refers to the communication link from the UEs to the base stations.
A UE may transmit a random access preamble on the uplink when the UE desires to gain access to the system. A base station may receive the random access preamble, and respond with a random access response that may contain related information for the UE. For example, the following events will trigger random access of a UE, i.e. RRC (Radio Resource Control) connection, RRC re-establishment, handover, UL data arrival when UL non-sync, and DL data arrival when UL non-sync. Furthermore, there are generally two kinds of random access, i.e. contention based random access and non-contention based random access. In a contention based random access, UE will select a random access preamble and RACH (Random Access Channel) itself, so there might be access collision if two UE choose the same random access preamble and RACH, which leads to the failure of the random access.
Although in a number of conventional wireless systems, usually only one UL carrier is available for one UE to perform the contention based random access on it, in some systems, there is a plurality of UL carriers available for a contention based random access of one UE. For example, in a system where an inter-band carrier aggregation is introduced, a random access only on the primary component carrier as in the intra-band carrier aggregation might not be enough. This is because that, multi-TA (Timing Advance) will be needed if the UL component carriers are in the different bands, and before a UE is synchronized to the network, there will be no other way to get timing advance on these component carriers but a random access. So multiple random accesses should be supported to measure the timing advance. That is to say, there may be more than one UL component carriers available for one UE to perform random access.
Then, when a UE desires to gain access to the system on one of the plurality of UL component carriers, for example there are few data to be transmitted without need of all UL component carriers, the UE will face more than one choices, i.e. choosing which UL component carrier to perform the contention based random access. Apparently, random selection will be the simplest way, but if the UE choose one UL component carrier with very high load of random access, it will cause a high failing rate of random access.
Therefore, it will be advantageous to provide a scheme for properly selecting a UL component carrier, to greatly improve the successful possibility of contention based random access and reduce the access delay.